Polishing film and method of producing same

ABSTRACT

A polishing film includes a plastic film and a polishing layer formed on its surface, having a mixture of a first group of silica particles with a first average diameter and a second group of silica particles with a second average diameter fixed in a resin binder, the first average diameter and the second diameter being different from each other and both within a range of 0.001-10 μm. The mixture of the two groups of silica particles has a granularity distribution curve with two peaks at two different diameter values corresponding to the first and second average diameters.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a polishing film for use in polishingthe surface of materials such as metals, ceramics, plastics and glass aswell as to a method of producing such a film. More particularly, thisinvention relates to a polishing film for the surface finishing ofprecision instruments such as optical fiber connectors forcommunications, color filters for liquid crystal displays, opticallenses, magnetic disk substrates and semiconductor wafers that require avery high degree of surface smoothness, as well as a method of producingsuch a polishing film.

[0002] If there are unwanted scratch marks or protrusions on the surfaceof such an instrument requiring a high degree of smoothness, theinstrument may cease to be able to function as expected according to itsdesign. Thus, the final polishing is a very important step that controlsthe functions of such a precision instrument. For example, opticalfibers for communications are coming to be commonly used ascommunication lines instead of the traditional copper wires, and theyare usually connected by means of connectors. Such optical fiberconnectors are formed by passing one or more optical fibers through thecenter of a ferrule made of partially stabilized zirconia obtained byadding aluminum to yttria and adhesively attached to the ferrule bymeans of a resin adhesive but their end surface must undergo a finalsurface polishing process such that the end parts of the optical fiberswill not protrude by more than 0.1 μm or indent by more than 0.05 μmfrom the end surface of the connector. If there is left at the end partof the optical fibers a damage or a left-over adhesive spilled out ofthe gap with the ferrule, or if the end part of the optical fibers ispolished excessively and becomes deformed, furthermore, scattering oflight and other phenomena take place at the end part of the opticalfibers and the transmission characteristics of the communication systemas a whole come to frustrate the original design. Thus, the end part ofsuch optical fibers must be smoothed to a high level of precision.

[0003] For the finishing of the end part of optical fibers and thepolishing of surfaces of other precision instruments, it has been knownto use a polishing tape having a polishing layer of abrading particles(very small spherical silica particles with average diameter of0.001-0.5 μm) and a resin binder formed on the surface of a plastic filmeither in the form of a tape or by cutting into another shape, asdisclosed, for example, in Japanese Patent Publications Tokkai 11-333731and 11-333732.

[0004] The surface smoothness of a precision-requiring component can beimproved by using such abrading particles with very small diameters soas to increase the contact surface of individual particles that act onthe surface to be polished. Since the polishing layer is extremely flat,however, there arises the problem of lowered polishing capability suchthat the throughput is adversely affected and locally unpolished spotsare left.

[0005] The polishing power can be increased while using abradingparticles with such very small diameters if the content of silicaparticles inside the polishing layer is increased such that the numberof abrading particles per unit area contacting the target surface to bepolished is increased. This, however, causes the contact area of theresin binder inside the polishing layer to be reduced, and this tends tocause the polishing layer to peel off locally during a polishingoperation. Such peeled-off portions of the polishing layer tend todamage the surface of the precision instrument, frustrating the effortto carry out a precise surface finishing process.

[0006] The polishing power can be increased also if relatively largerabrading particles are used. If the diameters of the abrading particlesare increased, however, the contact area of the resin binder in thepolishing layer with the plastic film increases and hence the problem ofthe polishing layer locally peeling off the plastic film can bealleviated but there arises instead the problem that the smoothness ofthe polished surface of the precision instrument is adversely affected.

[0007] As explained above, the smoothness of the polished surface of aprecision instrument, the polishing power and the phenomenon of thepolishing layer peeling off the plastic film are all dependent heavilyon the size of the abrading particles. Thus, the selection of the sizeof abrading particles is an important problem in the technical field ofsurface finishing of a precision instrument.

SUMMARY OF THE INVENTION

[0008] In view of the above, it is an object of this invention toprovide a polishing film capable of smoothly finishing the surface of aprecision instrument without leaving any localized unpolished spots.

[0009] It is another object of this invention to provide a method ofproducing such a polishing film.

[0010] A polishing film embodying this invention, with which the aboveand other objects can be accomplished may be characterized as comprisinga plastic film and a polishing layer formed on its surface, having amixture of a first group of silica particles with a first averagediameter and a second group of silica particles with a second averagediameter fixed in a resin binder wherein the first average diameter andthe second diameter are different from each other and both within arange of 0.001-10 μm.

[0011] The mixture of the two groups of silica particles according tothis invention has a granularity (grain size) distribution curve withtwo peaks at two different diameter values corresponding to the firstand second average diameters. In other words, silica particles havingtwo different diameters contribute mainly to the polishing and thesurface being polished therewith can be made into a smoothly finishedsurface with a high level of precision and without unpolished spots dueto a synergistic effect of these particles, unlike the conventionalpolishing films in which only particles with only one particulardiameter mainly contribute to the polishing.

[0012] For polishing the end surface of an optical fiber connector, itis preferred to use a polishing film using a mixture of a group ofsilica particles with average diameter of 0.02 μm and another group ofsilica particles with average diameter of 0.03 μm at a ratio of 9:1-6:4.

[0013] A method of producing a polishing film according to thisinvention may be characterized as comprising the steps of applying on asurface of a plastic film a paint obtained by dispersing in a resinbinder solution a mixture of silica particles as described above anddrying it to form a polishing layer on the surface of the plastic film.In this method, it is preferred to use a resin binder solution of a kindcontaining oligomers with siloxane bonds to form a network of cracks onthe surface of the polishing layer.

[0014] This invention therefore provides a polishing film in whichsilica particles with two different diameters contribute to thepolishing process and their synergistic effect makes it possible toobtain a smoothly polished surface without unpolished spots. Moreover,since silica particles with a relatively large diameter are fixed in thepolishing layer, the polishing layer does not peel off easily from theplastic film while being used in a polishing process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1(a) is a schematic sectional view of a polishing filmembodying this invention, and FIG. 1(b) is an enlarged plan view of aportion of the surface of its polishing layer.

[0016] FIGS. 2(a) and 2(b) are optical microscopic photographs showingthe condition of the end surface of an optical fiber connectorrespectively before and after it was polished by using a polishing filmof the test example in the comparison experiment.

[0017] FIGS. 3(a) and 3(b) are optical microscopic photographs showingthe condition of the end surface of an optical fiber connectorrespectively before and after it was polished by using a polishing filmof the comparison test example in the comparison experiment.

[0018]FIG. 4 is a schematic sectional view of an optical fiber connectorhaving its end surface polished in the comparison experiment.

[0019]FIG. 5 is a granularity distribution of silica particles withaverage diameter of 0.02 μm.

[0020]FIG. 6 is a granularity distribution of silica particles withaverage diameter of 0.03 μm.

[0021]FIG. 7 is a granularity distribution of the mixture of a group ofsilica particles with average diameter of 0.02 μm and another group ofsilica particles with average diameter of 0.03 μm of the test example.

DETAILED DESCRIPTION OF THE INVENTION

[0022]FIG. 1(a) shows a polishing film embodying this invention, havinga polishing layer 5 formed on the front surface of a plastic film 1. Thepolishing layer 5 is formed by fixing a mixture of a first group ofsilica particles 2 and a second group of silica particles 3 with a resinbinder 4 where the silica particles 2 of the first group has a (first)average diameter in the range of 0.001-10 μm and the silica particles 3of the second group has a different (second) average diameter also inthe same range. FIG. 7 shows the granularity distribution curve of thediameters of this mixture of silica particles, showing two peaks 23 and24 corresponding to the two different average diameter values for thefirst and second groups of silica particles 2 and 3.

[0023] In general, the granularity distribution of particles is obtainedby taking electron microscopic photographs at several arbitrarilyselected parts of the particles, counting the number of particles ineach range of their diameters and showing the result of the counting ina graph with the particle diameter shown along the horizontal axis andthe number or percentage of particles along the vertical axis. Theaverage diameter means where the peak appears on the distribution curve.Particles with the average diameter are most numerously contained andsuch particles having the average diameter mainly contribute in thepolishing process.

[0024] Let us assume, for example, that the first average diameter forthe first group of silica particles 2 is 0.02 μm and the second averagediameter for the second group of silica particles 3 is 0.03 μm. Withsilica particles of the first group alone, the distribution curve may befor example as shown in FIG. 5, having only one peak 21 at 0.02 μm andthose silica particles with diameter 0.02 μm contribute mainly in apolishing process. With silica particles of the second group alone, thedistribution curve may, be for example as shown in FIG. 6, having onlyone peak 22 at 0.03 μm and those silica particles with diameter 0.03 μmcontribute mainly in a polishing process. With a mixture of silicaparticles of both groups present, however, the distribution curve hastwo peaks 23 and 24 as shown in FIG. 7 at different diameter values 0.02μm and 0.03 μm and silica particles having two different diameter values0.02 μm and 0.03 μm contribute mainly in a polishing process. Accordingto the present invention, in summary, silica particles of two differentdiameter values are caused to contribute mainly in a polishing processsuch that the target surface of a precision instrument can be finishedinto a smooth surface with a reduced number of unpolished spots by asynergistic effect of silica particles of two groups with differentaverage diameters.

[0025] As an example of plastic film 1, use may be made of any knownflexible plastic film of a known kind having a large tensile strengthand a superior resistance against heat and chemicals such as films ofpolyethylene terephthalate (PET), polyester or polypropylene withthickness about 20-150 μm. In order to improve the adhesive force to thepolishing layer, use may be made of those with a surface on which aprimer processing of a known kind has been carried out.

[0026] The ratio of mixing silica particles of the first group and thesecond group 2 and 3 may be selected appropriately. If the mixing ratioof the group having the larger average diameter is increased, thepolishing power is increased to leave fewer unpolished spots and theprobability of the polishing layer 5 peeling off from the surface of theplastic film 1 becomes reduced.

[0027] When an end surface of an optical fiber connector is finished bypolishing, for example, the mixing ratio of silica particles with thesmaller average diameter with respect to those with the larger averagediameter should preferably be in the range of 9:1-6:4. This is becausethe tip portion of optical fibers may sustain an unwanted damage orbecome deformed if the mixing ratio of silica particles with the largeraverage diameter exceeds 50%. The ratio of the particles with the largeraverage diameter may be less than that of the particles with the smalleraverage diameter.

[0028] Polishing films according to this invention may be produced byapplying a paint material including aforementioned mixed particles and abinder on the surface of a plastic film and then drying it to therebyform a polishing layer on the surface of the plastic film. Examples ofthe binder which may be used for the purpose of this invention includeconventional polyurethane and polyester resin binders of known kinds.Methylethyl ketone may be used as its solvent. A hardening agent such asisocyanate may be added to a mixture of a resin binder and a solvent. Amixture of a resin binder and a solvent (and perhaps also a hardeningagent) will be hereinafter referred to as a “resin binder solution.” Themixing ratio of the mixed particles with respect to the resin bindersolution is within the range of 1:99-99:1

[0029] Oligomers such polyurethanes and polyesters having a siloxanebond may be used conveniently to form a network of cracks 6 on thesurface of the polishing layer 5 as shown in FIG. 1(b). These cracks 6serve to take in waste materials generated during a polishing processsuch that the waste materials can be prevented from damaging the targetsurface being polished.

[0030] The invention is described next by way of test and comparisonexamples. A polishing film as a test example was prepared by using amixture of silica particles of two groups with different averagediameters of 0.02 μm and 0.03 μm and having a distribution curve asshown in FIG. 7 with two peaks at 0.02 μm and 0.03 μm. Explained more indetail, a liquid having silica particles with average diameter 0.02 μmdispersed therein (IPA-ST by tradename produced by Nissan ChemicalIndustries Corporation) (3150 g) and another liquid having silicaparticles with average diameter 0.03 μm dispersed therein (MA-ST-M bytradename produced by Nissan Chemical Industries Corporation) (1012.5 g)were placed inside a mixer and this mixed liquid with silica particleswith different average diameters were mixed together for five minutes bystirring with ultrasonic waves. A resin binder solution comprisingoligomers having siloxane bonds (KR-211 by tradename produced byShinetsu Chemical Industries Corporation) (203.6 g) was gradually addedthereafter and mixed together by stirring for five minutes withultrasonic waves so as to uniformly disperse the mixed particles insidethis resin binder solution. Next, another resin binder solutioncomprising oligomers having siloxane bonds (T8001 by tradename producedby JSR Corporation) (37.5 g) was gradually added and mixed together bystirring for 30 minutes with ultrasonic waves so as to uniformlydisperse the mixed particles inside these resin binder solutions and tothereby obtain a paint.

[0031] After this paint was filtered through a 1.0 μm filter, it wasuniformly applied over a surface of a PET film of thickness 75 μm in anatmosphere of about 20° C. by a gravure reverse coating method, andafter it was dried so as to evaporate the solvent in the resin binder,it was further subjected to a heat process in an atmosphere of 100° C.to form a polishing layer of thickness 6 μm. In the above, the PET filmwas one obtained by extruding PET and polyester resin together and thensubjecting the extrusion to a drawing process. In other words, use wasmade of a PET film with a primer processing carried out thereon. Anetwork of cracks as shown in FIG. 1(b) was formed on the surface ofthis polishing layer.

[0032] For making a comparison with the test example described above,another polishing film of a comparison example was prepared, havingsilica particles with average diameter of 0.02 μm fixed in the sameresin binder as used for the test example. FIG. 5 shows the granularitydistribution of the silica particles, having a single peak at a diametervalue (0.02 μm) corresponding to their average diameter 0.02 μm.

[0033] Explained more in detail, a liquid having silica particles withaverage diameter 0.02 μm dispersed therein (IPA-ST by tradename producedby Nissan Chemical Industries Corporation) (4500 g) was placed inside amixer and this mixture was stirred together for five minutes withultrasonic waves. Next, as done for the test example, a resin bindersolution comprising oligomers having siloxane bonds (KR-211 by tradenameproduced by Shinetsu Chemical Industries Corporation) (203.6 g) wasgradually added and stirred for five minutes with ultrasonic waves so asto uniformly disperse the siloxane particles inside this resin bindersolution. Next, another resin binder solution comprising oligomershaving siloxane bonds (T8001 by tradename produced by JSR Corporation)(37.5 g) was gradually added and mixed together by stirring for 30minutes with ultrasonic waves so as to uniformly disperse the mixedparticles inside these resin binder solutions and to thereby obtain apaint.

[0034] After this paint was filtered through a 1.0 μm filter, it wasuniformly applied over a surface of a PET film of thickness 75 μm in anatmosphere of about 20° C. by a gravure reverse coating method, andafter it was dried so as to evaporate the solvent in the resin binder,it was further subjected to a heat process in an atmosphere of 100° C.to form a polishing layer of thickness 6 μm, as done for the textexample. A similar network of cracks as shown in FIG. 1(b) was formed onthe surface of this polishing layer.

[0035] As a comparison experiment, each of the polishing films of thetest and comparison examples was used to finish the end surface of anoptical fiber connector 10 as shown in FIG. 4 composed of acommunication optical glass fiber 11 penetrating and adhesively attachedto a zirconium ferrule 12 preliminarily roughly polished by using apolishing liquid containing diamond particles and a non-woven cloth pad.The polishing was carried out as shown in FIG. 4 by pasting a polishingfilm 15 on a rotary disk 14 having an elastic pad 13 attached theretoand pressing the end surface of the optical fiber connector with aspecified pressure while the disk 14 is rotated. The comparisonexperiment was carried out by rotating the disk 14 at the rate of 200rpm and pressing each fiber optical connector at a pressure of 180 g perfiber for 4 minutes.

[0036] If a polishing film of the test example is used, as can be seenin the optical microscopic photographs shown in FIGS. 2(a) and 2(b) thatthe resin (the dark black portions around the circular black part)spilling out from between the optical fiber (the circular black part)and the ferrule (the white part outside the circular black part) visiblebefore the finishing is removed and the end surface is polished smoothlywith a high level of precision. If a polishing film of the comparisonexample is used, by contrast, as can be seen in the optical microscopicphotographs shown in FIGS. 3(a) and 3(b) that the resin spilled out frombetween the optical fiber and the ferrule and visible before thefinishing process is not removed and remains visible, there beingunpolished spots left and a polishing with a high level of precision wasnot carried out.

What is claimed is:
 1. A polishing film comprising: a plastic film having a surface; and a polishing layer formed on said surface of said plastic film, said polishing layer having a mixture of a first group of silica particles with a first average diameter and a second group of silica particles with a second average diameter fixed in a resin binder, said first average diameter and said second diameter being different from each other and both within a range of 0.001-10 μm.
 2. The polishing film of claim 1 wherein said mixture has a granularity distribution curve with two peaks at two different diameter values corresponding to said first average diameter and said second average diameter.
 3. The polishing film of claim 1 which polishes an end surface of an optical fiber connector and has a surface with a network of cracks, wherein said resin binder includes oligomers with siloxane bonds.
 4. The polishing film of claim 3 wherein said mixture has a granularity distribution curve with two peaks at two different diameter values corresponding to said first average diameter and said second average diameter.
 5. A method of producing a polishing film, said method comprising the steps of: applying a paint on a surface of a plastic film, said paint being obtained by dispersing in a resin binder solution a mixture of a first group of silica particles with a first average diameter and a second group of silica particles with a second average diameter, said first average diameter and said second diameter being different from each other and both within a range of 0.001-10 μm; and drying said paint to form a polishing layer on said surface of said plastic film whereby said polishing film is produced.
 6. The method of claim 5 wherein said mixture has a granularity distribution curve with two peaks at two different diameter values corresponding to said first average diameter and said second average diameter.
 7. The method of claim 5 wherein said polishing film polishes an end surface of an optical fiber connector and has a surface with a network of cracks, and wherein said resin binder includes oligomers with siloxane bonds.
 8. The method of claim 7 wherein said mixture has a granularity distribution curve with two peaks at two different diameter values corresponding to said first average diameter and said second average diameter.
 9. The method of claim 5 wherein said resin binder includes oligomers with siloxane bonds.
 10. The method of claim 6 wherein said resin binder includes oligomers with siloxane bonds.
 11. The method of claim 7 wherein said resin binder includes oligomers with siloxane bonds.
 12. The method of claim 8 wherein said resin binder includes oligomers with siloxane bonds.
 13. The method of claim 5 wherein the step of drying includes the step of forming a network of cracks on said polishing layer.
 14. The method of claim 6 wherein the step of drying includes the step of forming a network of cracks on said polishing layer.
 15. The method of claim 7 wherein the step of drying includes the step of forming a network of cracks on said polishing layer.
 16. The method of claim 8 wherein the step of drying includes the step of forming a network of cracks on said polishing layer.
 17. The method of claim 9 wherein the step of drying includes the step of forming a network of cracks on said polishing layer.
 18. The method of claim 10 wherein the step of drying includes the step of forming a network of cracks on said polishing layer.
 19. The method of claim 11 wherein the step of drying includes the step of forming a network of cracks on said polishing layer.
 20. The method of claim 12 wherein the step of drying includes the step of forming a network of cracks on said polishing layer. 